1. Field of the Invention
The present invention pertains to multiple part cores for investment castings, and particularly to multiple part cores for hollow gas turbine engine blade castings, and methods for preparing such multiple part cores.
2. Discussion of the Related Art
Turbine blades for high performance gas turbine engines are generally required to have an internal cavity to provide a conduit for cooling air supplied to holes and slots distributed about the blades. Without such, the blades would not be able to operate in the high temperature environment where temperatures on the order of 2,800.degree. F. are commonplace, even when the blades are formed from modern, high temperature resistant superalloys such as the new "reactive" superalloys which have recently shown substantial benefits for advanced, single crystal gas turbine engine blade applications. See U.S. Pat. No. 4,719,080 (Duhl). As a consequence, conventional blade forming processes and apparatus use a separate core part for investment casting such blades, with the separate core part determining the internal cavity dimensions of the cast blade. Various core materials and core forming techniques are known in the art, and such are described, e.g., in U.S. Pat. No. 4,191,720 (Pasco et al.) and U.S. Pat. No. 4,532,974 (Mills et al.).
FIG. 1 shows a conventional one piece core for forming the internal cavity of a gas turbine engine blade and designated generally by the numeral 10. Core 10 has a portion 10a which determines the cavity dimensions in the "leading edge" portion of the cast blade, and a portion 10b determines the shape of its cavity in the "trailing edge" blade portion. In the core pictured in FIG. 1, the edge 13 of core portion 10b also determines the shape of the trailing edge slot of the cast blade. FIG. 2B represents schematically edge 13 of core 10 determinative of the trailing edge slot of the gas turbine blade and having a thickness dimension H.sub.0.
In operation of the gas turbine, it is important to accurately control the cooling air flow to various blade parts. Insufficient flow can result in "hot spots" leading to the possibility of early blade failure, and excess flow decreases the thermal performance of the engine. In general, it is advantageous to produce blades having the smallest trailing edge slot thickness that can be reliably and accurately maintained. In an effort to better control the cooling air flow out of the cast blade trailing edge slot and to increase the heat transferred to the cooling air, conventional cores are provided with an array of through-holes to allow the formation of pedestals in the cast product. The pedestals reinforce the trailing edge and provide a labyrinth-type flow restriction as well as increased blade internal surface for heat transfer. FIG. 2A shows such an array of pedestal-forming through-holes 20 having a pitch spacing S.sub.0.
To mold a complex ceramic core design similar to the one depicted in FIG. 1, the ceramic core molding material must first enter the mold cavity, fill the zones of least resistance, and then proceed to fill the zones of greatest resistance to flow. Those zones of greatest resistance to flow typically are those of the smallest cross sectional dimensions or those which possess a high surface area to volume ratio (i.e., long, thin trailing edge exits).
Ceramic core compositions utilizing thermoplastic binde materials such as those typically used in injection molding processes tend to resist flow and even solidify rapidly in constricted zones of core dies. If the runner feeding system does not solidify, the material pressure within the cavity builds to the hydraulic pressure applied on the material at the nozzle of the press. However, it has been a typical experience of injection molders that even when the maximum pressure is applied, the core die does not completely fill to form an acceptable article. This is especially true when attempting to produce cores with thin trailing edge exits. These exits are areas where the die surface area to mold volume aspect ratio is unfavorable from a heat transfer and flow standpoint. Consequently, conventional cores and core forming techniques result in blade products having minimum blade slot thickness dimensions greater than about 0.015 inches and minimum pedestal pitch spacing of greater than about 0.015 inches, on a commercially practicable basis.
Also, conventional one piece cores made by the various core manufacturing processes such as transfer molding and injection molding require relatively complex "multi-pull" dies of the oblique relationship between the axes of the pedestal-forming through-holes located near the trailing edge forming core portion and other through-holes proximate the leading edge core portion, such as the rib forming holes 20 in FIG. 1. This oblique relationship is due to blade (and thus core) curvature. Such complex dies can be quite costly and also can complicate the molding procedure.